Nanocomposites

ABSTRACT

Methods for preparing nanocomposites with electrical properties modified by powder size below 100 nanometers. Both low-loaded and highly-loaded nanocomposites are included. Nanoscale coated, un-coated, whisker type fillers are taught. Electrical nanocomposite layers may be prepared on substrates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/081,115, filed Apr. 10, 2008, which is a continuation of U.S.application Ser. No. 10/435,222, filed May 9, 2003, which is adivisional of U.S. application Ser. No. 09/790,036, filed Feb. 20, 2001,now U.S. Pat. No. 6,933,331, which is a continuation of U.S. applicationSer. No. 09/083,893, filed May 22, 1998, now U.S. Pat. No. 6,228,904,which claims priority to U.S. Provisional Application No. 60/079,225,filed Mar. 24, 1998, which also claims priority to U.S. ProvisionalApplication No. 60/069,935, filed Dec. 17, 1997, which also claimspriority to U.S. Provisional Application No. 60/049,077, filed Jun. 5,1997. Each application in this paragraph is incorporated by referenceherein.

FIELD OF THE INVENTION

This invention relates to the use of nanoscale powders as a component ofnovel composites and devices. By incorporating powders having dimensionsless than a characteristic domain size into polymeric and othermatrices, nanocomposites with unique properties can be produced.

BACKGROUND

A very wide variety of pure phase materials such as polymers are nowreadily available at low cost. However, low cost pure phase materialsare somewhat limited in the achievable ranges of a number of properties,including, for example, electrical conductivity, magnetic permeability,dielectric constant, and thermal conductivity. In order to circumventthese limitations, it has become common to form composites, in which amatrix is blended with a filler material with desirable properties.Examples of these types of composites include the carbon black andferrite mixed polymers that are used in toners, tires, electricaldevices, and magnetic tapes.

The number of suitable filler materials for composites is large, butstill limited. In particular, difficulties in fabrication of suchcomposites often arise due to issues of interface stability between thefiller and the matrix, and because of the difficulty of orienting andhomogenizing filler material in the matrix. Some desirable properties ofthe matrix (e.g., rheology) may also be lost when certain fillers areadded, particularly at the high loads required by many applications. Theavailability of new filler materials, particularly materials with novelproperties, would significantly expand the scope of manufacturablecomposites of this type.

SUMMARY

Briefly stated, the present invention is directed to nanocomposites andproducts wherein the presence of novel nanofillers enhance a wide rangeof properties. In another aspect, the present invention is directed tomethods for preparing nanocomposites that enable nanotechnologyapplications offering advantages such as superior processability(rheology), thermal conductivity, thermal robustness, electricalconductivity, optical clarity and superior functional performance. In anexample method, nanofillers and a substance having a polymer are mixed.Both low-loaded and highly-loaded nanocomposites are contemplated.Nanoscale coated and un-coated fillers may be used. Nanocomposite filmsmay be coated on substrates.

In one aspect, the invention comprises a nanostructured filler,intimately mixed with a matrix to form a nanostructured composite. Atleast one of the nanostructured filler and the nanostructured compositehas a desired material property which differs by at least 20% from thesame material property for a micron-scale filler or a micron-scalecomposite, respectively. The desired material property is selected fromthe group consisting of refractive index, transparency to light,reflection characteristics, resistivity, permittivity, permeability,coercivity, B-H product, magnetic hysteresis, breakdown voltage, skindepth, curie temperature, dissipation factor, work function, band gap,electromagnetic shielding effectiveness, radiation hardness, chemicalreactivity, thermal conductivity, temperature coefficient of anelectrical property, voltage coefficient of an electrical property,thermal shock resistance, biocompatibility and wear rate.

The nanostructured filler may comprise one or more elements selectedfrom the s, p, d, and f groups of the periodic table, or it may comprisea compound of one or more such elements with one or more suitableanions, such as aluminum, antimony, boron, bromine, carbon, chlorine,fluorine, germanium, hydrogen, indium, iodine, nickel, nitrogen, oxygen,phosphorus, selenium, silicon, sulfur, or tellurium. The matrix may be apolymer (e.g., poly(methyl methacrylate), poly(vinyl alcohol),polycarbonate, polyalkene, or polyaryl), a ceramic (e.g., zinc oxide,indium-tin oxide, hafnium carbide, or ferrite), or a metal (e.g.,copper, tin, zinc, or iron). Loadings of the nanofiller may be as highas 95%, although loadings of 80% or less are preferred. The inventionalso comprises devices which incorporate the nanofiller (e.g.,electrical, magnetic, optical, biomedical, and electrochemical devices).

Another aspect of the invention comprises a method of producing acomposite, comprising blending a nanoscale filler with a matrix to forma nanostructured composite. Either the nanostructured filler or thecomposite itself differs substantially in a desired material propertyfrom a micron-scale filler or composite, respectively. The desiredmaterial property is selected from the group consisting of refractiveindex, transparency to light, reflection characteristics, resistivity,permittivity, permeability, coercivity, B-H product, magnetichysteresis, breakdown voltage, skin depth, curie temperature,dissipation factor, work function, band gap, electromagnetic shieldingeffectiveness, radiation hardness, chemical reactivity, thermalconductivity, temperature coefficient of an electrical property, voltagecoefficient of an electrical property, thermal shock resistance,biocompatibility, and wear rate. The loading of the filler does notexceed 95 volume percent, and loadings of 80 volume percent or less arepreferred.

The composite may be formed by mixing a precursor of the matrix materialwith the nanofiller, and then processing the precursor to form a desiredmatrix material. For example, the nanofiller may be mixed with amonomer, which is then polymerized to form a polymer matrix composite.In another embodiment, the nanofiller may be mixed with a matrix powdercomposition and compacted to form a solid composite. In yet anotherembodiment, the matrix composition may be dissolved in a solvent andmixed with the nanofiller, and then the solvent may be removed to form asolid composite. In still another embodiment, the matrix may be a liquidor have liquid like properties.

Many nanofiller compositions are encompassed within the scope of theinvention, including nanofillers comprising one or more elementsselected from the group consisting of actinium, aluminum, arsenic,barium, beryllium, bismuth, cadmium, calcium, cerium, cesium, cobalt,copper, dysprosium, erbium, europium, gadolinium, gallium, gold,hafnium, hydrogen, indium, iridium, iron, lanthanum, lithium, magnesium,manganese, mendelevium, mercury, molybdenum, neodymium, neptunium,nickel, niobium, osmium, palladium, platinum, potassium, praseodymium,promethium, protactinium, rhenium, rubidium, scandium, silver, sodium,strontium, tantalum, terbium, thallium, thorium, tin, titanium,tungsten, vanadium, ytterbium, yttrium, zinc, and zirconium.

“Domain size” as that term is used herein, refers to the minimumdimension of a particular material morphology. In the case of powders,the domain size is the grain size. In the case of whiskers and fibers,the domain size is the diameter. In the case of plates and films, thedomain size is the thickness.

As used herein, a “nanostructured powder” is one having a domain size ofless than 100 nm, or alternatively, having a domain size sufficientlysmall that a selected material property is substantially different fromthat of a micron-scale powder, due to size confinement effects (e.g.,the property may differ by 20% or more from the analogous property ofthe micron-scale material). Nanostructured powders often advantageouslyhave sizes as small as 50 nm, 30 nm, or even smaller. Nanostructuredpowders may also be referred to as “nanopowders” or “nanofillers.” Ananostructured composite is a composite comprising a nanostructuredphase dispersed in a matrix.

As it is used herein, the term “agglomerated” describes a powder inwhich at least some individual particles of the powder adhere toneighboring particles, primarily by electrostatic forces, and“aggregated” describes a powder in which at least some individualparticles are chemically bonded to neighboring particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described with reference to the several figures of thedrawings, in which:

FIG. 1 is a diagram of a nanostructured filler coated with a polymer;

FIG. 2 portrays an X-ray diffraction (XRD) spectrum for thestoichiometric indium tin oxide powder of Example 1;

FIG. 3 is a scanning electron microscope (SEM) micrograph of thestoichiometric indium tin oxide powder of Example 1; and

FIG. 4 is a diagram of the nanostructured varistor of Example 5.

DETAILED DESCRIPTION

Prior art filler materials for polymeric composites are usually powderswith an average dimension in the range of 10-100 μm. Thus, each fillerparticle typically has on the order of 10¹⁵-10¹⁸ atoms. In contrast thetypical polymer chain has on the order of 10³-10⁹ atoms. While the artof precision manufacturing of polymers at molecular levels iswell-developed, the knowledge of precision manufacturing of fillermaterials at molecular levels has remained largely unexplored.

The number of atoms in the filler particles of the invention(hereinafter called “nanostructured filler” or “nanofiller”) is on theorder of or significantly less than the number of atoms in the polymermolecules, e.g., 10²-10¹⁰. Thus, the filler particles are comparable insize or smaller than the polymer molecules, and therefore can bedispersed with orders of magnitude higher number density. Further, thefillers may have a dimension less than or equal to the critical domainsizes that determine the characteristic properties of the bulkcomposition; thus, the fillers may have significantly different physicalproperties from larger particles of the same composition. This in turnmay yield markedly different properties in composites using nanofillersas compared to the typical properties of conventional polymercomposites.

These nanostructured filler materials may also have utility in themanufacture of other types of composites, such as ceramic- ormetal-matrix composites. Again, the changes in the physical propertiesof the filler particles due to their increased surface area andconstrained domain sizes can yield changes in the achievable propertiesof composites.

The nanofillers of the invention can be inorganic, organic, or metallic,and may be in the form of powders, whiskers, fibers, plates or films.The fillers represent an additive to the overall composite composition,and may be used at loadings of up to 95% by volume. The fillers may haveconnectivity in 0, 1, 2, or 3 dimensions. Fillers may be produced by avariety of methods, such as those described in U.S. Pat. Nos. 5,486,675;5,447,708; 5,407,458; 5,219,804; 5,194,128; and 5,064,464. Particularlypreferred methods of making nanostructured fillers are described in U.S.patent application Ser. No. 09/046,465, by Bickmore, et al., filed Mar.23, 1998, now U.S. Pat. No. 5,984,997 and Ser. No. 08/706,819, byPirzada, et al., filed Sep. 3, 1996, now U.S. Pat. No. 5,851,507 both ofwhich are incorporated herein by reference.

A wide variety of nanofiller compositions are possible. Some exemplarycompositions include metals (e.g., Cu, Ag, Ni, Fe, Al, Pd, and Ti),oxide ceramics (e.g., TiO₂, TiO_(2-x), BaFe₂O₄, dielectric compositions,ferrites, and manganites), carbide ceramics (e.g., SiC, BC, TiC, WC,WC_(1-x)), nitride ceramics (e.g., Si₃N₄, TiN, VN, AlN, and Mo₂N),hydroxides (e.g., aluminum hydroxide, calcium hydroxide, and bariumhydroxide), borides (e.g., AlB₂ and TiB₂), phosphides (e.g., NiP andVP), sulfides (e.g., molybdenum sulfide, titanium sulfide, and tungstensulfide), silicides (e.g., MoSi₂), chalcogenides (e.g., Bi₂Te₃, Bi₂Se₃),and combinations of these.

The fillers are immediately mixed with a matrix material, which ispreferably polymeric, but may also be ceramic, metallic, or acombination of the above. The matrix may be chosen for properties suchas ease of processability, low cost, environmental benignity, commercialavailability, and compatibility with the desired filler. The fillers arepreferably mixed homogeneously into the matrix, but may also be mixedheterogeneously if desired, for example to obtain a composite having agradient of some property. Mixing techniques for incorporating powdersinto fluids and for mixing different powders are well known in the art,and include mechanical, thermal, electrical, magnetic, and chemicalmomentum transfer techniques, as well as combinations of the above.

The viscosity, surface tension, and density of a liquid matrix materialcan be varied for mixing purposes, the preferred values being those thatfavor ease of mixing and that reduce energy needed to mix withoutintroducing any undesirable contamination. One method of mixing is todissolve the matrix in a solvent which does not adversely affect theproperties of the matrix or the filler and which can be easily removedand recovered. Another method is to melt the matrix, incorporate thefiller, and cool the mixture to yield a solid composite with the desiredproperties. Yet another method is to synthesize the matrix in-situ withthe filler present. For example, the nanofiller can be mixed with aliquid monomer, which can then be polymerized to form the composite. Inthis method, the filler may be used as a catalyst or co-catalyst forpolymerization. The mixing may also be accomplished in the solid state,for example by mixing a powdered matrix composition with the filler, andthen compacting the mixture to form a solid composite.

Mixing can be assisted using various secondary species such asdispersants, binders, modifiers, detergents, and additives. Secondaryspecies may also be added to enhance one to more of the properties ofthe filler-matrix composite.

Mixing can also be assisted by pre-coating the nanofiller with a thinlayer of the matrix composition or with a phase that is compatible withthe matrix composition. Such a coated nanoparticle is illustrated inFIG. 1, which shows a spherical nanoparticle 6 and a coating 8. In oneembodiment, when embedding nanofillers in a polymer matrix, it may bedesirable to coat the filler particles with a related monomer. Whenmixing nanofillers into a ceramic matrix, pre-coating can be done byforming a ceramic layer around the nanoscale filler particle during orafter the synthesis of the nanoscale filler, by methods such as partialoxidation, nitridation, carborization, or boronation. In these methods,the nanostructured filler is exposed to a small concentration of aprecursor that reacts with the surface of the filler to form a ceramiccoating. For example, a particle may be exposed to oxygen in order tocreate an oxide coating, to ammonia in order to create a nitridecoating, to borane to create a boride coating, or to methane to create acarbide coating. It is important that the amount of precursor be small,to prevent thermal runaway and consequent conversion of thenanostructured filler into a ceramic particle.

In case of polymer matrix, the filler can be coated with a polymer or amonomer by numerous methods, for example, surface coating in-situ, spraydrying a dispersion of filler and polymer solution, co-polymerization onthe filler surface, and melt spinning followed by milling. A preferredmethod is surface coating in-situ. In this process, the filler is firstsuspended in demineralized water (or another solvent) and thesuspension's pH is measured. The pH is then adjusted and stabilized withsmall addition of acid (e.g., acetic acid or dilute nitric acid) or base(e.g., ammonium hydroxide or dilute sodium hydroxide). The pH adjustmentproduces a charged state on the surface of the filler. Once a desired pHhas been achieved, a coating material (for example, a polymer or otherappropriate precursor) with opposite charge is introduced into thesolvent. This step results in coupling of the coating material aroundthe nanoscale filler and formation of a coating layer around thenanoscale filler. Once the layer has formed, the filler is removed fromthe solvent by drying, filtration, centrifugation, or any other methodappropriate for solid-liquid separation. This technique of coating afiller with another material using surface charge can be used for avariety of organic and inorganic compositions.

When a solvent is used to apply a coating as in the in-situ surfacecoating method described above, the matrix may also be dissolved in thesolvent before or during coating, and the final composite formed byremoving the solvent.

A very wide range of material properties can be engineered by thepractice of the invention. For example, electrical, magnetic, optical,electrochemical, chemical, thermal, biomedical, and tribologicalproperties can be varied over a wider range than is possible using priorart micron-scale composites.

Nanostructured fillers can be used to lower or raise the effectiveresistivity, effective permittivity, and effective permeability of apolymer or ceramic matrix. While these effects are present at lowerloadings, they are expected to be most pronounced for filler loadings ator above the percolation limit of the filler in the matrix (i.e., atloadings sufficiently high that electrical continuity exists between thefiller particles). Other electrical properties which may be engineeredinclude breakdown voltage, skin depth, Curie temperature, temperaturecoefficient of electrical property, voltage coefficient of electricalproperty, dissipation factor, work function, band gap, electromagneticshielding effectiveness and degree of radiation hardness. Nanostructuredfillers can also be used to engineer magnetic properties such as thecoercivity, B-H product, hysteresis, and shape of the B-H curve of amatrix.

An important characteristic of optical material is its refractive indexand its transmission and reflective characteristics. Nanostructuredfillers may be used to produce composites with refractive indexengineered for a particular application. Gradient lenses may be producedusing nanostructured materials. Gradient lenses produced fromnanostructured composites may reduce or eliminate the need for polishinglenses. The use of nanostructured fillers may also help filter specificwavelengths. Furthermore, a key advantage of nanostructured fillers inoptical applications is expected to be their enhanced transparencybecause the domain size of nanostructured fillers ranges from about thesame as to more than an order of magnitude less than visible wavelengthsof light.

The high surface area and small grain size of nanofilled composites makethem excellent candidates for chemical and electrochemical applications.When used to form electrodes for electrochemical devices, thesematerials are expected to significantly improve performance, for exampleby increasing power density in batteries and reducing minimum operatingtemperatures for sensors. (An example of the latter effect can be foundin copending and commonly assigned U.S. application Ser. No. 08/739,257,“Nanostructured Ion Conducting Solid Electrolytes,” by Yadav, et al. nowU.S. Pat. No. 5,905,000). Nanostructured fillers are also expected tomodify the chemical properties of composites. These fillers arecatalytically more active, and provide more interface area forinteracting with diffusive species. Such fillers may, for example,modify chemical stability and mobility of diffusing gases. Furthermore,nanostructured fillers may enhance the chemical properties ofpropellants and fuels.

Many nanostructured fillers have a domain size comparable to the typicalmean free path of phonons at moderate temperatures. It is thusanticipated that these fillers may have dramatic effects on the thermalconductivity and thermal shock resistance of matrices into which theyare incorporated.

Nanostructured fillers—in coated and uncoated form—and nanofilledcomposites are also expected to have significant value in biomedicalapplications for both humans and animals. For example, the small size ofnanostructured fillers may make them readily transportable through poresand capillaries. This suggests that the fillers may be of use indeveloping novel time-release drugs and methods of administration anddelivery of drugs, markers, and medical materials. A polymer coating canbe utilized either to make water-insoluble fillers into a form that iswater soluble, or to make water-soluble fillers into a form that iswater insoluble. A polymer coating on the filler may also be utilized asa means to time drug-release from a nanoparticle. A polymer coating mayfurther be used to enable selective filtering, transfer, capture, andremoval of species and molecules from blood into the nanoparticle.

A nanoparticulate filler for biomedical operations might be a carrier orsupport for a drug of interest, participate in the drug's functioning,or might even be the drug itself. Possible administration routes includeoral, topical, and injection routes. Nanoparticulates and nanocompositesmay also have utility as markers or as carriers for markers. Theirunique properties, including high mobility and unusual physicalproperties, make them particularly well-adapted for such tasks.

In some examples of biomedical functions, magnetic nanoparticles such asferrites may be utilized to carry drugs to a region of interest, wherethe particles may then be concentrated using a magnetic field.Photocatalytic nanoparticles can be utilized to carry drugs to region ofinterest and then photoactivated. Thermally sensitive nanoparticles cansimilarly be utilized to transport drugs or markers or species ofinterest and then thermally activated in the region of interest.Radioactive nanoparticulate fillers may have utility for chemotherapy.Nanoparticles suitably doped with genetic and culture material may beutilized in similar way to deliver therapy in target areas.Nanocomposites may be used to assist in concentrating the particle andthen providing the therapeutic action. To illustrate, magnetic andphotocatalytic nanoparticles may be formed into a composite,administered to a patient, concentrated in area of interest usingmagnetic field, and finally activated using photons in the concentratedarea. As markers, nanoparticulate fillers—coated or uncoated—may be usedfor diagnosis of medical conditions. For example, fillers may beconcentrated in a region of the body where they may be viewed bymagnetic resonance imaging or other techniques. In all of theseapplications, the possibility exists that nanoparticulates can bereleased into the body in a controlled fashion over a long time period,by implanting a nanocomposite material having a bioabsorbable matrix,which slowly dissolves in the body and releases its embedded filler.

As implants, nanostructured fillers and composites are expected to lowerwear rate and thereby enhance patient acceptance of surgical procedures.Nanostructured fillers may also be more desirable than micron-scalefillers, because the possibility exists that their domain size may bereduced to low enough levels that they can easily be removed by normalkidney action without the development of stones or other adverse sideeffects. While nanoparticulates may be removed naturally through kidneyand other organs, they may also be filtered or removed externallythrough membranes or otherwise removed directly from blood or tissue.Carrier nanoparticulates may be reactivated externally through membranesand reused; for example, nutrient carriers may be removed from thebloodstream, reloaded with more nutrients, and returned to carry thenutrients to tissue. The reverse process may also be feasible, whereincarriers accumulate waste products in the body, which are removedexternally, returning the carriers to the bloodstream to accumulate morewaste products.

EXAMPLES Example 1 Indium Tin Oxide Fillers in PMMA

A stoichiometric (90% wt % In₂O₃ in SnO₂) indium tin oxide (ITO)nanopowder was produced using the methods of copending patentapplication Ser. No. 09/046,465. 50 g of indium shot was placed in 300ml of glacial acetic acid and 10 ml of nitric acid. The combination, ina 1000 ml Erlenmeyer flask, was heated to reflux while stirring for 24hours. At this point, 50 nil of HNO₃ was added, and the mixture washeated and stirred overnight. The solution so produced was clear, withall of the indium metal dissolved into the solution, and had a totalfinal volume of 318 ml. An equal volume (318 mL) of 1-octanol was addedto the solution along with 600 mL ethyl alcohol in a 1000 mL HDPEbottle, and the resulting mixture was vigorously shaken. 11.25 ml oftetrabutyltin was then stirred into the solution to produce a clearindium/tin emulsion. When the resulting emulsion was burned in air, itproduced a brilliant violet flame. A yellow nanopowder residue wascollected from the flamed emulsion. The nanopowder surface area was 13.5m²/gm, and x-ray diffractometer mean grain size was 60 nm.

FIG. 2 shows the measured X-ray diffraction (XRD) spectrum for thepowder, and FIG. 3 shows a scanning electron microscope (SEM image ofthe powder. These data show that the powder was of nanometer scale.

The nanostructured powder was then mixed with poly(methyl methacrylate)(PMMA) in a ratio of 20 vol % powder to 80 vol % PMMA. The powder andthe polymer were mixed using a mortar and pestle, and then separatedinto three parts, each of which was pressed into a pellet. The pelletswere pressed by using a Carver hydraulic press, pressing the mixtureinto a ¼ inch diameter die using a 1500 pound load for one minute.

After removal from the die, the physical dimensions of the pellets weremeasured, and the pellets were electroded with silver screen printingpaste (Electro Sciences Laboratory 9912-F).

Pellet resistances were measured at 1 volt using a Megohmmeter/IR tester1865 from QuadTech with a QuadTech component test fixture. The volumeresistivity was calculated for each pellet using the standard relation,

$\rho = {R\left( \frac{A}{t} \right)}$

where ρ represents volume resistivity in ohm-cm, R represents themeasured resistance in ohms, A represents the area of the electrodedsurface of the pellet in cm², and t represents the thickness of thepellet in cm. The average volume resistivity of the stoichiometric ITOcomposite pellets was found to be 1.75×10⁴ ohm-cm.

Another quantity of ITO nanopowder was produced as described above, andwas reduced by passing 2 SCFM of forming gas (5% hydrogen in nitrogen)over the powder while ramping temperature from 25° C. to 250° C. at 5°C./min. The powder was held at 250° C. for 3 hours, and then cooled backto room temperature. The XRD spectrum of the resulting powder indicatedthat the stoichiometry of the reduced powder was In₁₈SnO_(29-x), with xgreater than 0 and less than 29.

The reduced ITO nanopowder was combined with PMMA in a 20:80 volumeratio and formed into pellets as described above. The pellets wereelectroded as described, and their resistivity was measured. The averageresistivity for the reduced ITO composite pellets was found to be1.09×10⁴ ohm-cm.

For comparison, micron scale ITO was purchased from Alfa Aesar (catalognumber 36348), and was formed into pellets with PMMA and electroded asdescribed above. Again, the volume fraction of ITO was 20%. The averagemeasured resistivity of the micron scale ITO composite pellets was foundto be 8.26×10⁸ ohm-cm, representing a difference of more than fourorders of magnitude from the nanoscale composite pellets. It was thusestablished that composites incorporating nanoscale fillers can haveunique properties not achievable by prior art techniques.

Example 2 Hafnium Carbide Fillers in PMMA

Nanoscale hafnium carbide fillers were prepared as described incopending U.S. patent application Ser. Nos. 08/706,819 and 08/707,341.The nanopowder surface area was 53.5 m²/gm, and mean grain size was 16nm. Micron scale hafnium carbide powder was purchased from Cerac(catalog number H-1004) for comparison.

Composite pellets were produced as described in Example 1, by mixingfiller and polymer with a mortar and pestle and pressing in a hydraulicpress. Pellets were produced containing either nanoscale or micron scalepowder at three loadings: 20 vol % powder, 50 vol % powder, and 80 vol %powder. The pellets were electroded as described above, and theirresistivities were measured. (Because of the high resistances at the 20%loading, these pellets' resistivities were measured at 100V. The otherpellets were measured at IV, as described in Example 1).

Results of these resistivity measurements are summarized in Table 1. Ascan be seen, the resistivity of the pellets differed substantiallybetween the nanoscale and micron scale powders. The compositesincorporating nanoscale powder had a somewhat decreased resistivitycompared to the micron scale powder at 20% loading, but had adramatically increased resistivity compared to the micron scale powderat 50% and 80% loading.

TABLE 1 Volume % Resistivity of nanoscale Resistivity of micron scalefiller powder composite (ohm-cm) powder composite (ohm-cm) 20 5.54 ×10¹² 7.33 × 10¹³ 50 7.54 × 10⁹ 2.13 × 10⁴ 80 3.44 × 10⁹ 1.14 × 10⁴

Example 3 Copper Fillers in PMA and PVA

Nanoscale copper powders were produced as described in U.S. patentapplication Ser. Nos. 08/706,819 and 08/707,341. The nanopowder surfacearea was 28.1 m²/gm, and mean grain size was 22 nm. Micron scale copperpowder was purchased from Aldrich (catalog number 32645-3) forcomparison.

The nanoscale and micron scale copper powders were each mixed at aloading of 20 vol % copper to 80 vol % PMMA and formed into pellets asdescribed above. In addition, pellets having a loading of 15 vol %copper in poly(vinyl alcohol) (PVA) were produced by the same method.The pellets were electroded and resistivities measured at 1 volt asdescribed in Example 1. Results are shown in Table 2.

TABLE 2 Volume % Volume Resistivity Additive Polymer filler (ohm-cm)nanoscale copper PMMA 20 5.68 × 10¹⁰ nanoscale copper PVA 15 4.59 × 10⁵micron scale copper PMMA 20 4.19 × 10¹²

It can be seen from Table 2 that the resistivity of the nanoscale copperpowder/PMMA composite was substantially reduced compared to the micronscale copper powder/PMMA composite at the same loading, and that theresistivity of the nanoscale copper powder/PVA composite was lower stillby five orders of magnitude.

Example 4 Preparation of Polymer-Coated Nanostructured Filler

The stoichiometric (90 wt % In₂O₃ in SnO₂) indium tin oxide (ITO)nanopowder of Example 1 was coated with a polymer as follows.

200 milligrams of ITO nanopowders with specific surface area of 53 m²/gmwere added to 200 ml of demineralized water. The pH of the suspensionwas adjusted to 8.45 using ammonium hydroxide. In another container, 200milligrams of poly(methyl methacrylate) (PMMA) was dissolved in 200 mlof ethanol. The PMMA solution was warmed to 100° C. while being stirred.The ITO suspension was added to the PMMA solution and the stirring andtemperature of 100° C. was maintained till the solution reduced to avolume of 200 ml. The solution was then cooled to room temperature to avery homogenous solution with very light clear-milky color. The opticalclarity confirmed that the powders are still nanostructured. The powderwas dried in oven at 120° C. and its weight was measured to be 400milligrams. The increase in weight, uniformity of morphology and theoptical clarity confirmed that the nanopowders were coated with PMMApolymer.

The electrochemical properties of polymer coated nanopowders weredifferent than the as-produced nanopowders. The as-produced nanopowderwhen suspended in demineralized water yielded a pH of 3.4, while thepolymer coated nanopowders had a pH of 7.51.

Example 5 Preparation of Electrical Device Using Nanostructured Fillers

A complex oxide nanoscale filler having the following composition wasprepared: Bi₂O₃ (48.8 wt %), NiO (24.4 wt %), CoO (12.2 wt %), Cr₂O₃(2.4 wt MnO (12.2 wt %), and Al₂O₃ (<0.02 wt %). The complex oxidefiller was prepared from the corresponding nitrates of the same cation.The nitrates of each constituent were added to 200 mL of deionized waterwhile constantly stirring. Hydroxides were precipitated with theaddition of 50 drops of 28-30% NH₄OH. The solution was filtered in alarge buchner funnel and washed with deionized water and then with ethylalcohol. The powder was dried in an oven at 80° C. for 30 minutes. Thedried powder was ground using a mortar and pestle. A heat treatmentschedule consisting of a 15° C./min ramp to 350° C. with a 30 minutedwell was used to calcine the ground powder.

The nanofiller was then incorporated at a loading of 4% into a zincoxide ceramic matrix. The composite was prepared by mechanically mixingthe doped oxide nanofiller powder with zinc oxide powder, incorporatingthe mixture into a slurry, and screen printing the slurry (furtherdescribed below). For comparison, devices were made using both ananoscale matrix powder produced by the methods of copending andcommonly assigned U.S. application Ser. No. 08/706,819, and using amicron scale matrix powder purchased from Chemcorp. The fillers and thematrix powders were mixed mechanically using a mortar and pestle.

Using the filler-added micron scale powder, a paste was prepared bymixing 4.0 g of powder with 2.1 g of a commercial screen printingvehicle purchased from Electro Science Laboratories (ESL vehicle 400).The doped nanoscale powder paste was made using 3.5 g powder and 3.0 gESL vehicle 400. Each paste was mixed using a glass stir rod.Silver-palladium was used as a conducting electrode material. A screenwith a rectangular array pattern was used to print each paste on analumina substrate. First a layer of silver-palladium powder (the lowerelectrode) was screen printed on the substrate and dried on a hot plate.Then the ceramic filled powder was deposited, also by screen printing.Four print-dry cycles were used to minimize the possibility of pinholedefects in the varistor. Finally, the upper electrode was deposited.

The electrode/composite/electrode varistor was formed as threediagonally offset overlapping squares, as illustrated in FIG. 4. Theeffective nanostructured-filler based composite area in the device dueto the offset of the electrodes was 0.036 in² (0.2315 cm²). The greenthick films were co-fired at 900° C. for 60 minutes. The screen printedspecimen is shown in FIG. 4, where light squares 10 represent thesilver-palladium electrodes, and dark square 12 represents the compositelayer.

Silver leads were attached to the electrodes using silver epoxy. Theepoxy was cured by heating at a 50° C./min ramp rate to 600° C. and thencooling to room temperature at a rate of 50° C./min. The TestPointcomputer software, in conjunction with a Keithley® current source, wasused to obtain a current-voltage curve for each of the varistors.Testpoint and Keithley are trademarks or registered trademark ofKeithley Scientific Instruments, Inc.

The electrode/micron scale matrix composite/electrode based varistordevice had a total thickness of 29-33 microns and a composite layerthickness of 19 microns. The electrode/nanoscale matrixcomposite/electrode based varistor device had a total thickness of 28-29microns and a composite layer thickness of 16 microns. Examination ofcurrent-voltage response curves for both varistors showed that thenanostructured matrix varistor had an inflection voltage of about 2volts, while the inflection voltage of the micron scale matrix varistorhad an inflection voltage of about 36 volts. Fitting the current-voltageresponse curves to the standard varistor power-law equation

l=nV^(a)  (2)

yielded values of voltage parameter a of 2.4 for the micron-scale matrixdevice, and 37.7 for the nanoscale matrix device. Thus, the nonlinearityof the device was shown to increase dramatically when the nanoscalematrix powder was employed.

Example 6 Thermal Battery Electrode Using a Nanostructured Filler

Thermal batteries are primary batteries ideally suited for militaryordinance, projectiles, mines, decoys, torpedoes, and space explorationsystems, where they are used as highly reliable energy sources with highpower density and extremely long shelf life. Thermal batteries havepreviously been manufactured using techniques that place inherent limitson the minimum thickness obtainable while ensuring adequate mechanicalstrength. This in turn has slowed miniaturization efforts and haslimited achievable power densities, activation characteristics, safety,and other important performance characteristics. Nanocomposites helpovercome this problem, as shown in the following example.

Three grams of raw FeS₂ powder was mixed and milled with a group of hardsteel balls in a high energy ball mill for 30 hours. The grain size ofproduced powder was 25 nm. BET analysis showed the surface area of thenanopowder to be 6.61 m²/gm. The TEM images confirmed that the ballmilled FeS₂ powder consists of the fine particles with the round shape,similar thickness and homogenous size. The cathode comprised FeS₂nanopowders (68%), eutectic LiCl—KCl (30%) and SiO₂ (2%) (from AldrichChemical with 99% purity). The eutectic salts enhanced the diffusion ofLi ions and acted as a binder. Adding silicon oxide particles wasexpected to immobilize the LiCl—KCl salt during melting. For comparison,the cathode pellets were prepared from nanostructured and micron scaleFeS₂ powders separately.

To improve electrochemical efficiencies and increase the melting pointof anode, we chose micron scale Li 44%-Si 56% alloy with 99.5% purity(acquired from Cyprus Foote Mineral) as the anode material in this work.A eutectic salt, LiCl 45%-KCl 55% (from Aldrich Chemical with 99%purity), was selected as electrolyte. The salt was dried at 90° C. andfused at 500° C. To strengthen the pellets and prevent flowing out ofelectrolyte when it melted, 35% MgO (Aldrich Chemical, 99% purity)powder was added and mixed homogeneously with the eutectic salt powder.

The pellets of anode electrodes were prepared by a cold press process. Ahard steel die with a 20 mm internal diameter was used to make the thindisk pellets. 0.314 grams of Li 44%-Si 56% alloy powder (with 76-422mesh particle size) was pressed under 6000 psi static pressure to form apellet. The thickness and density of the pellets so obtained wasdetermined to be 0.84 mm and 1.25 g/cm², respectively. Electrolytepellets were produced using 0.55 grams of blended electrolyte powderunder 4000 psi static pressure. The thickness and density of the pelletsobtained were 0.84 mm and 2.08 g/cm² respectively. The cathode pelletwas prepared using 0.91 grams of mixed micron scale FeS₂—LiCl—KCl—SiO₂powder pressed under 4000 psi static pressure. The thickness and densityof the pellets obtained were 0.86 mm and 3.37 g/cm², respectively.

A computerized SOLARTRON® 1287 electrochemical interface and a 1260Gain/Phase Analyzer were employed to provide constant current and tomonitor variation in potential between anode and cathode of cells duringthe discharging. “Solartron” is a registered trademark of the SolartronElectronic Group, Ltd. The cutoff potential of discharge was set at 0.8volt. The thermal battery with the nanocomposite cathode provided 1 Aconstant current for 246 seconds, until the potential fell to 0.8 volt.It was observed that the power density of the nanostructured single cellthermal battery was 100% higher than that achievable with micron sizedmaterials. Thus, nanoscale fillers can help enhance the electrochemicalperformance of such a device.

Example 7 A Magnetic Device Using Nanostructured Ferrite Fillers

Ferrite inductors were prepared using nanostructured and micron-scalepowders as follows. One-tenth of a mole (27.3 grams) of iron chloridehexahydrate (FeCl₃-6H₂O) was dissolved in 500 ml of distilled wateralong with 0.025 moles (3.24 grams) of nickel chloride (NiCl₂) and 0.025moles (3.41 grams) of zinc chloride (ZnCl₂). In another large beaker, 25grams of NaOH was dissolved in 500 ml of distilled water. While stirringthe NaOH solution rapidly, the metal chloride solution was slowly added,forming a precipitate instantaneously. After 1 minute of stirring, theprecipitate solution was vacuum filtered while frequently rinsing withdistilled water. After the precipitate had dried enough to cake andcrack, it was transferred to a glass dish and allowed to dry for 1 hourin an 80° C. drying oven. At this point, the precipitate was ground witha mortar and pestle and calcined in air at 400° C. for 1 hour to removeany remaining moisture and organics.

BET analysis of the produced powder yielded a surface area of 112 m²/g,confirming the presence of nanometer-sized individual particles with anestimated BET particle size of 11 nm. XRD analyses of all nanoscalepowders showed the formation of a single (Ni, Zn)Fe₂O₄ ferrite phasewith peak shapes characteristic of nanoscale powders. XRD peakbroadening calculations reported an average crystallite size of 20 nm ofthe thermally quenched powders and 8 nm for the chemically derivedpowders. SEM-EDX analyses of sintered nanopowder pellets showed anaverage composition of 14.8% NiO, 15.8% ZnO, and 69.4% Fe₂O₃, whichcorresponded to the targeted stoichiometric composition of theNi_(0.5)Zn_(0.5)Fe₂O₄.

Nanoscale ferrite filler powders were uniaxially pressed at 5000 poundsin a quarter-inch diameter die set into green pellets. The powders weremixed with 2 weight percent Duramax® binder for improved sinterability.The amount of powder used for pressing varied from 1.5 to 1.7 grams,typically resulting in cylinders having a post-sintered height ofapproximately 1.5 cm. To avoid cracking and other thermal stresseffects, a multi-level heating profile was employed. The pellets werefired at a rate of 5° C./min to 300° C., 10° C./min to 600° C., and 20°C./min to the final sintering temperature, where it was held for fourhours. Pellets were cooled from the sintering temperature at a rate of10° C./min to ensure the sintering temperature ranged from 900° C. to1300° C., but was typically greater than 1200° C. to ensure anacceptable density. Sintered pellets were then wound with 25 turns of 36gauge enamel coated wire, the wire ends were stripped, and the completedsolenoids where used for electrical characterization. An air coil wasprepared for the purpose of calculating magnetic properties. This coilwas created by winding 25 turns of the enamel coated wire around the dieplunger used previously. This coil was taped with masking tape, slid offthe plunger slowly to maintain shape and characteristics, and wascharacterized along with the ferrite solenoids.

Inductance characterization was performed with a Hewlett-Packard 429A RFImpedance/Materials Analyzer. Impedance, parallel inductance, q factor,and impedance resistance were measured over a logarithmic frequencysweep starting at 1 MHz and ending at 1.8 GHz. Values for permeability(μ) and loss factor (LF) were calculated from inductance (L), air coilinductance (L₀), and impedance resistance (R) using the followingequations:

$\begin{matrix}{{\mu = \frac{L}{L_{0}}}{{LF} = \frac{L_{0}R}{\omega \; L^{2}}}} & \;\end{matrix}$

Resistivity measurements were made with a Keithley® 2400 SourceMeterusing a four-wire probe attachment and TestPoint™ data acquisitionsoftware. Voltage was ramped from 0.1 to 20 volts while simultaneouslymeasuring current. The results were plotted as field (voltage divided bypellet thickness) versus current density (current divided by electrodecross sectional area). The slope of this graph gives materialresistivity (ρ).

Table 3 summarizes electrical properties of inductors prepared frommicron-sized powder or from nanopowder. In most cases there is anadvantage to using nanoscale precursor powder instead of micron-sizedpowder. It is important to keep in mind that all measurements were takenfrom cylindrical devices, which have inherently inefficient magneticproperties. Solenoids of this shape were used in this study because ofthe ease of production and excellent reproducibility. All measuredproperties would be expected to improve with the use of higher magneticefficiency shapes such as cores or toroids, or by improving the aspectratio (length divided by diameter) of the cylindrical samples.

TABLE 3 Micron Nano Loss Factor @ 1 MHz Average 0.0032 0.0025 Q Factor @1 MHz Average 37.2 52.2 Critical Frequency Average 68.9 MHz 78.3 MHzResistivity Average 0.84 MΩ 33.1 MΩ 

The inductors made from ferrite nanopowders exhibited significantlyhigher Q-factor, critical resonance frequency, and resistivity. Theyalso exhibited more than 20% lower loss factor as is desired incommercial applications.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of the specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

1. A method for preparing a nanocomposite comprising: mixing nanofillerswith a polymer matrix, wherein the nanofillers have a domain size equalto or less than the mean free path of phonons; processing the mixture toproduce a solid nanocomposite having a nanoparticle loading of 20% to80% by volume in the solid polymer matrix; and wherein the nanocompositehas an electrical conductivity that differs by more than 20% as comparedwith the electrical conductivity exhibited by a composite material ofthe same composition with filler particles having a domain size of 10microns.
 2. The method of claim 1, wherein the nanofillers have a domainsize equal to or less than 100 nanometers.
 3. The method of claim 1,wherein the mixing includes adding a secondary species material selectedfrom the group consisting of: dispersants, binders, modifiers,detergents, and additives.
 4. The method of claim 1, wherein thenanofillers are heterogeneously dispersed in the solid polymer matrix.5. The method of claim 1, further comprising depositing thenanocomposite on a substrate.
 6. The method of claim 1, furthercomprising applying the nanocomposite as a layer over another substance.7. The method of claim 1, wherein the nanofillers comprise oxide.
 8. Themethod of claim 1, wherein the nanofillers comprise nitride.
 9. Themethod of claim 1, wherein the nanofillers comprise whiskers.
 10. Themethod of claim 1, wherein the nanofillers comprise one or more elementsselected from the group consisting of: aluminum, barium, bismuth,cadmium, calcium, cerium, cesium, cobalt, copper, europium, gallium,gold, indium, iron, lanthanum, lithium, magnesium, manganese,molybdenum, neodymium, nickel, niobium, palladium, platinum, potassium,praseodymium, scandium, silver, sodium, strontium, tantalum, tin,titanium, tungsten, vanadium, ytterbium, yttrium, zinc, and zirconium.11. The method of claim 1, wherein the nanofillers comprise one or moreelements selected from the group consisting of: antimony, boron,bromine, carbon, chlorine, fluorine, germanium, hydrogen, iodine,nitrogen, oxygen, phosphorus, selenium, silicon, sulfur, and tellurium.12. The method of claim 1, further comprising coating the nanofillerswith one of a monomer or a polymer before mixing the nanofillers withthe polymer matrix.
 13. The method of claim 1, wherein the nanocompositehas an electrical conductivity and a thermal conductivity that differ bymore than 20% as compared with the electrical conductivity and thermalconductivity exhibited by a composite material of the same compositionwith filler particles having a domain size of 10 microns.
 14. The methodof claim 1, wherein processing the mixture to produce a solidnanocomposite comprises polymerizing a mixture of nanofiller andmonomer.
 15. The method of claim 1, wherein processing the mixture toproduce a solid nanocomposite comprises compacting a mixture ofnanofiller and polymer powder.
 16. The method of claim 1, whereinprocessing the mixture to produce a solid nanocomposite comprisesremoving solvent from a mixture of nanofiller and polymer.
 17. Themethod of claim 1, wherein processing the mixture to produce a solidnanocomposite comprises solidifying a mixture of nanofiller and liquidpolymer.
 18. A nanocomposite layer prepared using the method of claim 1.19. A device prepared using the method of claim
 1. 20. A productprepared using the method of claim 1.